Oxytitanium phthalocyanine is known to exhibit several different crystal forms in addition to .alpha.- and .beta.-crystal forms reported in JP-A-63-20365 and JP-A-63-20366 (the term "JP-A" as used herein means an "unexamined published Japanese patent application"). Of the known crystal forms, those showing the maximum peak in their X-ray diffraction patterns at a Bragg angle (2.theta..+-.0.2.degree.) of 27.3.degree. have been attracting particular attention for use as a charge generating material of electrophotographic photoreceptors because of their high photosensitivity. For example, the oxytitanium phthalocyanine disclosed in JP-A-62-67094, which has the maximum diffraction peak at a Bragg angle (2.theta..+-.0.2.degree.) of 27.3.degree., exhibits very high sensitivity. However, these species proved insufficient in stability on repeated use, stability of the crystal form in a coating composition, and dispersibility in a coating composition.
Processes for obtaining oxytitanium phthalocyanine crystals having the maximum diffraction peak at a Bragg angle (2.theta..+-.0.2.degree. C.) of 27.3.degree. include, in addition to the process disclosed in JP-A-62-67094 supra, a process comprising a general acid paste process followed by a solvent treatment as disclosed in JP-A-63-20365 and JP-A-1-299874, a process comprising heating or milling in an organic solvent in the presence of sulfuric acid or sulfonic acid as disclosed in JP-A-2-215866 and JP-A-2-215867, and a process comprising milling for making oxytitanium phthalocyanine amorphous, followed by a methanol treatment at room temperature or under heating as disclosed in JP-A-3-54264 and JP-A-3-62039. However, any of the crystals obtained is poor in crystal form stability in a solvent and easily converted to a more stable form.
JP-A-64-17066 discloses a process in which oxytitanium phthalocyanine is milled together with sodium chloride and polyethylene glycol. However, the resulting crystal form is easily converted to other crystal forms depending on temperature because of use of a solvent. Moreover, the particle size is difficult to control, resulting in non-uniformity in particle size. JP-A-2-289658 discloses a process comprising mechanically grinding oxytitanium phthalocyanine until it is converted into amorphous or the X-ray diffraction results in substantially no peak or no more than a slight peak at a Bragg angle (2.theta..+-.0.2.degree.) of 27.3.degree., and then treating the powder with a solvent. However, mechanical grinding to make the crystal show a peak at a Bragg angle (2.theta..+-.0.2.degree.) of 27.3.degree. requires a very long time and is not suited for industrial production. In addition, the resulting crystals are non-uniform in particle size, and it is difficult to obtain in a stable manner an oxytitanium phthalocyanine hydrate crystal doped with a specific amount of water of crystallization.